The present invention relates to a passenger detector, which is used to detect the presence of a passenger on the seat of a vehicle and/or said passenger""s position on said seat. Such a detector has a particular application in controlling a motor vehicle protection system such as airbags.
In order to protect the life of passengers during a traffic accident, modern vehicles are generally fitted with a protection system comprising several airbags and seat belt pretensioners, which are used to absorb the energy of a passenger released during the collision due to the accident. In order to deploy the airbags of the passenger seat only in the case where this seat is actually occupied by a person, passenger detectors have been developed that indicate the presence of a passenger to the control unit of the protection system.
Such a detector is described, for example, in the document DE-A-42 37 072. It concerns a pressure detector integrated into the passenger seat, which comprises two insulating substrates placed one above the other and separated by a certain distance using a separator. The separator comprises, for example, a two-sided adhesive band which is cut in such a way as to surround at least in part the active zones of the detector. Inside the active zones, one of the supporting sheets is provided with two electrode structures made of a conducting material separated from each other, while the other supporting sheet is provided with a coating of a pressure-sensitive semiconducting material. The semiconducting material has microprojections on the surface so that the surface resistance between the layer and a conductor decreases when the layer is pressed on to the conductor.
When the passenger seat is unoccupied, i.e. when no pressure is acting on the passenger detector, the layer of semiconducting material is not in contact with the two electrodes and the electrical resistance between the two electrodes is consequently very high. If, on the contrary, a person is sitting on the seat, the two supporting sheets are pressed together and the pressure-sensitive layer is put into contact with the two electrodes. This produces, between the two electrodes, a short circuit whose electrical resistance varies inversely with the value of the applied pressure. The greater the pressure on the sensor, the more the semiconducting layer is compressed or the more it comes into intimate contact with the electrodes, and the more the resistance measured between the two electrodes decreases.
The document DE 197 38 531 describes a resistance varying with pressure comprising a first substrate and a second substrate. A layer of conducting material is printed on the first substrate, which carries insulating particles having a diameter greater than the thickness of the layer. On the second support two electrodes are arranged at a distance from each other. The two substrates are then laminated together, the insulating particles forming separators to prevent contact between the conducting layer and the electrodes when no pressure is exerted on the sensor.
Another laminated pressure sensor is described in the document WO-A-97/18450. It concerns a sensor for measuring the pressure of a foot, comprising piezoelectric sensors inserted in a flexible structure as in a sandwich
One disadvantage of such a detector lies in its high rigidity. In effect, due to the lamination of the two supporting sheets using an adhesive band, the detector is quite thick and has a high resistance to bending or twisting, which significantly reduces its flexibility. Consequently, the introduction of such a detector in the seat of a vehicle may have a detrimental effect on the comfort of the seat.
Another type of force sensor is described in the document xe2x80x9cRobotics and power semiconductors join forces to meet the challenges of the automated factory environmentxe2x80x9dxe2x80x94Electronic Design, vol. 31, no 10, May 1983 (1983-05), pages 97-111, XP002092415, Waseca, Minn., Denville, N.J., USA. This document describes a force sensor comprising two electrode structures placed one on top of the other. The first of these electrode structures is placed on a silicon substrate, while the second is formed by a conducting elastomer. Placed between these two structures is an insulating layer separating the two electrodes when no force is acting on the sensor. It is clear that, due to its rigid support, such a sensor is in no way suitable for being integrated into a car seat.
The objective of the present invention is to propose a passenger detector that does not have this disadvantage.
This objective is attained by a passenger detector comprising a flexible support of an insulating material, at least two electrode structures placed on said insulating substrate at a distance from each other, and a layer of semiconducting material placed on the top of said electrode structures in an active zone of the detector. In conformity with the invention, said layer of semiconducting material has an internal resistance that varies with a deformation of said layer, and said layer of semiconducting material is placed in intimate contact with said electrode structures.
It should be noted that the internal resistance of the semiconducting layer may vary not only with a compression of the layer but also with a bending or any other deformation of the layer. Moreover, the internal resistance of the layer may vary in the same sense as the deformation (e.g. the resistance increases when the pressure increases) or in the opposite sense of the deformation (e.g. the resistance decreases when the pressure increases).